Abstract
A regular temporal pattern sets up the expectation that the pattern will continue at the same pace. Because of this expectation, we easily notice random, intermittent changes in a regular pattern, e.g., ‘a missing beat’. Here we use high-density EEG to determine whether early visual areas only encode the physical stimulus or also contain representations of the expected sensory input. In a dynamic random dot stereogram, subjects viewed a 5 deg disparity-defined disk that repetitively (1 Hz) moved forward and back 6 arcmin from an annular surround (12° diameter) presented in the fixation plane. Intermittently (30% of time), the pattern changed; either the disparity step was doubled to 12 arcmin or was omitted. The subjects were asked to detect these changes in the pattern. We compared responses from trials where changes, whether doubled or omitted, were correctly identified (hits) with responses from trials in which no change occurred, again correctly identified (correct rejects) within several fMRI-defined ROI's in visual cortex. In V3A and LOC, the early response, consisting of a peak at about 200 ms, reflected the magnitude of the disparity modulation. It was absent when the step was omitted, present for the regular step and enhanced when the step was doubled. However, later components of the response to the detected intermittent changes in modulation, i.e. the hits, were very similar. By time-locking the response analysis to the button press we determined that these later components occurred at a fixed timing relationship to the subject's response. Our results suggest that areas V3A and LOC may encode a mismatch between the expected temporal pattern and the actual sensory input, permitting the subject to correctly identify the change and that activity in these areas is causally related to the timing of the decision as reflected by the button press latency.